Pituitary Adenylate Cyclase-Activating Polypeptide (“PACAP”) is a member of the secretin/vasoactive intestinal peptide (“VIP”)/growth hormone-releasing hormone (“GHRH”) family. PACAP is a multifunctional vasodilatory peptide that exists in two α-amidated active forms, one with 38 amino acids (PACAP38; SEQ ID NO: 1241) and the other with 27 amino acids (PACAP27; SEQ ID NO: 1242). Both peptides have the same N-terminal 27 amino acids and are synthesized from the same precursor protein, preproPACAP (See, Moody et al., Curr. Opin. Endocrinol. Diabetes Obes., 18(1): 61-67, 2011). PACAP38 is the more prevalent active form, representing up to 90% of PACAP forms in mammalian tissues (See, Kaiser and Russo, Neuropeptides, 47:451-461, 2013). The sequence of PACAP38 is identical in all mammals and differs from the avian and amphibian orthologs by only one amino acid (See, Vaudry et al., Pharmacol. Rev., 52:269-324, 2000). The secretin/VIP/GHRH family includes mammalian peptide histidine methioneamide (“PHM”), secretin, glucagon, glucagon-like peptide-1 (“GLP1”), glucagon-like peptide-2 (“GLP2”), glucose-dependent-insulinotrophic-polypeptide (“GIP”), and growth-hormone-releasing-factor (“GRF”). PACAP27 has 68% sequence identity to VIP at the amino acid level (See, Vaudry et al. 2000).
PACAP is widely distributed in the brain and peripheral organs, e.g., the endocrine system, gonads, sympathetic neurons, respiratory system, gastrointestinal tract, cardiovascular system, and urogenital tracts (See, Schytz et al., Neurotherapeutics, 7:191-196, 2010). In particular, PACAP is expressed throughout the nervous system, including a presence in the trigeminovascular system, trigeminal ganglia, spinal cord, hypothalamus, and pituitary. PACAP has roles in neurodevelopment, neuroprotection, neuromodulation, neurogenic inflammation, and nociception with multiple actions (See, Kaiser and Russo (2013)).
Consistent with its widespread distribution, PACAP exerts pleiotropic effects including modulation of neurotransmitter release, vasodilation, bronchodilation, and activation of intestinal motility, increase of insulin and histamine secretion, as well as stimulation of cell proliferation and/or differentiation. PACAP has been shown to act as a hormone, a neurohormone, a neurotransmitter, and a trophic factor in a number of tissues (See, Vaudry et al., Pharmacological Rev., 52(2):269-324, 2000).
The biological effects of PACAP are mediated via three different G-protein coupled receptors: PAC1-R, vasoactive intestinal peptide receptor type 1 (“VPAC1-R”), and vasoactive intestinal peptide receptor type 2 (“VPAC2-R”). These receptors are expressed in diverse tissues. PAC1-R is particularly abundant in the nervous system (e.g., olfactory bulb, thalamus, hypothalamus, cerebellum, and spinal dorsal horn), pituitary, and adrenal glands. By contrast, VPAC1-R and VPAC2-R are expressed mainly in the lung, liver, and testis, although they have been detected in other tissues as well. VPAC1-R expression has been detected in the nervous system (e.g., cerebral cortex and hippocampus), smooth muscle cells of lung, liver, intestine, megakaryocytes, and platelets. VPAC1-R associates with receptor-associated membrane protein (“RAMP”, specifically RAMP2) (See, Christopoulos et al., J. Biol. Chem., 278:3293-3297, 2002). VPAC2-R expression profile includes the nervous (e.g., thalamus, hippocampus, brain stem, and dorsal root ganglia (“DRG”)), cardiovascular system, gastrointestinal system, pancreas, and reproductive systems (See, Usdin et al., Endocrin., 135:2662-2680, 1994; Sheward et al., Neurosci., 67:409-418, 1995).
PAC1-R is selective for PACAP38 and PACAP27. In particular, PAC1-R binds to PACAP with 100-1000-fold greater affinity than VIP, i.e., KD˜0.5 nM for PACAP27/PACAP38 vs. KD˜500 nM for VIP. Conversely, VPAC1-R and VPAC2-R have equal affinities for PACAP and VIP (KD˜1 nM) (See Schytz et al. (2010)).
Upon activation, these receptors are all capable of causing downstream production of cyclic adenosine monophosphate (“cAMP”), and/or activation of phospholipase C (“PLC”), and/or modulation of phospholipase D (“PLD”). In particular, PAC1-R is coupled to dual signal transduction pathways acting through cAMP and Ca2+, whereas VPAC1-R and VPAC2-R are coupled principally to adenylyl cyclase. PAC1-R is coupled to Gs protein, which activates adenylyl cyclase to form cAMP that in turn activates protein kinase A. PAC1-R also couples to Gq and thereby activates PLC, which produces inositol phosphate, which increases cytosolic calcium release from intra-cellular calcium stores. There is some evidence for a role of PAC1-R in PLD activation (See McCulloch et al., Ann. N. Y. Acad. Sci., 921:175-185, 2000). Another PACAP signaling pathway results in the elevation of intra-cellular sodium levels via activation of nonselective cation channels (See Roy et al., American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 304(12):R1070-R1084, 2013).
PACAP is hypothesized to play a role in a multitude of diseases and disorders, including but not limited to migraine, headache, and pain, though such a role for PACAP has not been clinically demonstrated. Migraines are believed to have a neurovascular component. Migraines affect approximately 10% of the adult population in the U.S. and are typically accompanied by intense headaches. Approximately 20-30% of migraine sufferers experience aura, comprising focal neurological phenomena that precede and/or accompany the event. A role for PACAP in migraine has been suggested by several observations: (1) plasma levels of PACAP are elevated during migraine attacks (ictal), as compared to interictal levels, in humans (see, Tuka et al., Cephalalgia, 33(13):1085-1095, 2013); (2) an infusion of PACAP38 triggered headaches in healthy subjects, and headaches followed by migraine-like attacks in migraineurs (see, Schytz et al., Brain, 132:16-25 2009; and Amin et al., Brain, 137:779-794, 2014, respectively); (3) PACAP-induced vasodilation may play a role in neurogenic inflammation (see, Kaiser and Russo, Neuropeptides, 47:451-461, 2013); and (4) PACAP-induced migraines are associated with photophobia, phonophobia, nausea, and respond to triptans (see, Amin et al., Brain, 32:140-149 2012). PACAP has also been shown to induce vasodilation, photophobia, as well as mast cell degranulation and neuronal activation (See, Markovics et al., Neurobiology of Disease, 45:633-644 2012; Baun et al., Cephalalgia, 32(4):337-345, 2012; Chan et al., Pharmacology & Therapeutics, 129:332-351, 2011).
One effective treatment for migraines is the administration of triptans, which are a family of tryptamine-based drugs, including sumatriptan and rizatriptan. Members of this family have an affinity for multiple serotonin receptors, including 5-HT1B, 5-HT1D, and 5-HT1F. Members of this family of drugs selectively constrict cerebral vessels, but also cause vasoconstrictive effects on coronary vessels (See Durham, New Eng. J. Med., 350 (11):1073-75, 2004). There is a theoretical risk of coronary spasm in patients with established heart disease following administration, and cardiac events after taking triptans in rare instances may occur. Accordingly, they are contraindicated for some patients with coronary vascular disease.
Similarly, pain may often be addressed through the administration of certain narcotics or non-steroidal anti-inflammatory drugs (“NSAIDs”). However, the administration of these treatments often has negative consequences. NSAIDs have the potential to cause kidney failure, intestinal bleeding, and liver dysfunction. Narcotics have the potential to cause nausea, vomiting, impaired mental functioning, and addiction. Therefore, it is desirable to identify alternative treatments for pain in order to avoid certain of these negative consequences.
PACAP may also be involved in diseases and disorders other than migraine, headache, and pain. For example, PACAP may correlate to or even play a causal role in anxiety disorders (WO 2012/106407); thrombocytopenia (WO 2004/062684); and inflammatory skin diseases (WO 2010/007175). PACAP and PAC1-R polymorphisms are associated with post-traumatic stress syndrome (“PTSD”) in females, major depressive disorder, and generalized anxiety disorder, suggesting a role for PACAP in these conditions. Further, supporting a role for PACAP in thrombocytopenia, trisomy 18 patients have excess PACAP and exhibit defective megakaryocyte maturation (See, Schytz et al. 2010; and Moody et al., Curr. Opin. Endocrinol. Diabetes Obes., 18(1):61-67, 2011).
Also, PACAP and other neuropeptides, such as Calcitonin Gene-Related Peptide (“CGRP”), substance P, neurokinin A, bradykinin, and endothelin-1, are expressed in the lower urinary tract (“LUT”) (see, Arms and Vizzard, Handbook Exp. Pharmacol., 202:395-423 2011) and reportedly may play a role in LUT dysfunction and urinary tract disorders such as urinary tract infection (“UTI”), abnormal voiding, urinary urgency, nocturia, urinary incontinence, overactive bladder, and the pain associated with such conditions.
PACAP and PACAP receptors have also been suggested to modulate inflammatory and neuropathic pain and have been implicated in both pronociception and antinociception (See, Davis-Taber et al., J. Pain, 9(5):449-56 2008). PACAP has also been reported to be required for spinal desensitization and the induction of neuropathic pain (See, Mabuchi et al., J. Neurosci., 24(33):7283-91, 2004). Additionally, morphine withdrawal behavior is reportedly modified in PACAP-receptor deficient mice further suggesting the role of PACAP in morphine withdrawal anxiolytic response (See, Martin et al., Mol. Brain Res., 110(1):109-18 2003).